Semiconductor device

ABSTRACT

A layer in which the potential level difference normally unrequired for device operation is generated is positively inserted in a device structure. The potential level difference has such a function that even if a semiconductor having a small bandgap is exposed on a mesa side surface, a potential drop amount of the portion is suppressed, and a leakage current inconvenient for device operation can be reduced. This effect can be commonly obtained for a heterostructure bipolar transistor, a photodiode, an electroabsorption modulator, and so on. In the photodiode, since the leakage current is alleviated, the device size can be reduced, so that in addition to improvement of operating speed with a reduction in series resistance, it is advantageous that the device can be densely disposed in an array.

TECHNICAL FIELD

The present invention relates to a device structure of a semiconductordevice.

BACKGROUND ART

A so-called compound semiconductor device manufactured by using a groupIII-V compound semiconductor is used as a heterostructure bipolartransistor (HBT) excellent in high-speed operation or various opticaldevices having a light-emitting function, a light-receiving function,and a light-modulating function and is a component essential to acurrent optical communication system and a wireless system. In such acompound semiconductor device, as the device is miniaturized to requirehigher-speed operation, the following problems due to the compoundsemiconductor easily occur.

In an integrated-circuit technology based on a Si-based material, it ispossible to take advantage of a fabrication process technology having adegree of freedom, such as thermal diffusion of impurities, ionimplantation, Si oxidation/insulation layer formation, poly-SIdeposition, and selective growth. Meanwhile, the compound semiconductordevice often has a device configuration based on a mesa structure. Thus,in the compound semiconductor device, there are large restrictions dueto problems of the controllability of dimensions associated with mesaprocessing, the material characteristics, and surface characteristics.Especially, it is difficult to inactivate a surface of the compoundsemiconductor device (i.e. passivation). The reason why it is difficultto perform passivation is that a level is generated on a surface of thecompound semiconductor. The level generated on the surface traps acarrier charge to toughen a control of a potential distribution in adevice structure, and, thus, to generate an abnormal forward currentassociated with a current path of the surface, whereby there occurs aproblem that the level becomes a recombination center and increases adark current.

The problem of the surface characteristics of the compound semiconductorsignificantly affects when InGaAs with a high electron mobility mostsuitable for the high-speed operation is used in a base layer and acollector layer of HBT. Further, the problem of the surfacecharacteristics of the compound semiconductor also affects InGaAs and anoptical device using a multiple quantum well structure containing InGaAsaccording to the wavelength (1.5 micron band) used in long-distanceoptical communication. This is because the bandgap energy of InGaAs isso small as 0.75 eV, and, at the same time, passivation is difficult;therefore, a leakage current in the pn junction tends to increase. Theproblem of the surface characteristics of the compound semiconductor iscommon to a heterostructure bipolar transistor, a pin-type photodiode,and so on.

FIG. 6 is a cross-sectional view for explaining a structure of aconventional typical ultrafast HBT 50. In the HBT 50, the structureshould be miniaturized to realize the high speed, and usually, a deviceis constituted by stacking mesa type pn junctions. The HBT 50 is of annpn type in which InGaAs is used in a p-type base layer 506 and ann-type collector layer 503.

In the HBT 50, an InP sub-collector layer 502 electrically separated inan island-shaped manner is disposed on a semi-insulating InP substrate501, and a base-collector mesa constituted of a low concentration ofn-type InGaAs collector layer 503 and a low concentration of p-typeInGaAs base layer 506 is disposed on the InP sub-collector layer 502,and an n-type InP emitter layer 507 is disposed on the mesa. The HBT 50is further provided with an emitter electrode 508, a base electrode 509,and a collector electrode 510. Usually, as in the HBT 50, the p-typebase layer 506 and the n-type collector layer 503 are mesas having thesame size, and a band diagram in an A-A′ cross section of FIG. 6 isshown in FIG. 7. Thus, an electrical field remains as it is on a mesaside surface of a base-collector junction. In the base-collectorjunction constituted of InGaAs, passivation of a side surface of themesa is difficult, and a leakage current in the junction tends toincrease. When the forward current increases, the on-voltage of thecollector increases, and thus there occurs a problem that operation in alow collector voltage region becomes difficult, and the reliability isimpaired due to the instability thereof. When a backward leakage currentis large, a portion of a base current flows to the collector side, andtherefore, such a problem that a low current operation becomes difficultmay occur.

As in a photodiode 60 of FIG. 8 to be described later, a layerequivalent to a non-doped InGaAsP surface cover layer 604 reducing theleakage current may be inserted under the p-type base layer 506.However, this structure increases the size of a collector mesa and mayhamper the miniaturization of the device.

FIG. 8 is a cross-sectional view for explaining a structure of aphotodiode for communication in which an ultrafast operation is requiredand, in particular, the photodiode 60 in which a speed of not less than10 Gb/s is required. In the photodiode 60, a structure of a mesa-typesemiconductor layer is often provided on a semi-insulating InP 601 forthe purpose of reducing a junction capacity.

The photodiode 60 is constituted of a mesa-processed layer in which ann-type InP contact layer 602, a low concentration of InGaAslight-absorbing layer 603, a low concentration of InGaAsP surface coverlayer 604, and a p-type InP contact layer 605 are provided in this orderfrom the lower layer side, and the photodiode 60 further has ap-electrode 606 and an n-electrode 607 required for voltage application.

Unlike the HBT 50 of FIG. 6, in the photodiode 60, a relatively narrowp-type region (p-type InP contact layer 605) is formed in anisland-shaped manner on a wide intermediate mesa including the InGaAslight-absorbing layer 603. An active region of a pn junction is a regiondefined by the p-type InP contact layer 605, and the junction capacityis reduced. FIG. 9(A) is a band diagram in an A-A′ cross section of FIG.8. As shown in the band diagram, the InGaAs light-absorbing layer 603 isdepleted to induce the magnetic field suitable for diode operation.

An upper surface of the InGaAs light-absorbing layer 603 is covered witha non-doped InGaAsP surface cover layer 604 having a larger bandgappreventing exposure of InGaAs. In order to enlarge the upper surface ofthe InGaAs light-absorbing layer 603, the intermediate mesa is widercompared with a conventional photodiode. By virtue of such a structure,the electric field extending into the side surface of the intermediatemesa is reduced, and the photodiode 60 can suppress an occurrence of aleakage current attributable to a surface of InGaAs.

There has been proposed an inverted photodiode structure in which thepolarity of the conductive type of the structure shown in FIG. 8 isswitched, a p-type InP contact layer is disposed in the lower portion,and an n-type InP contact layer is disposed in the upper portion (forexample, see Patent Document 1). The photodiode disclosed in the PatentDocument 1 can suppress the occurrence of a leakage current by similarreasoning.

In the structure of the photodiode 60 of FIG. 8, an electroabsorptionmodulator having a ridge waveguide is one in which the InGaAslight-absorbing layer 603 is replaced with an optical core layerincluding InGaAs in an inner structure. As in the case of the photodiode60, the electroabsorption modulator can suppress the occurrence of aleakage current attributable to an InGaAs surface.

PRIOR ART DOCUMENTS Patent Documents

Patent Literature 1: Japanese Patent Laid-Open Publication No.2010-147177

SUMMARY OF INVENTION Problems to be Solved by the Invention

As described above, a pn junction of a semiconductor material having asmall bandgap, such as InGaAs is used in various electronic devices andoptical devices. However, in a device for the purpose of high-speedoperation, even if the device has a structure similar to that of thephotodiode of FIG. 8 or the above electroabsorption modulator, it isdifficult to suppress the occurrence of a leakage current for thefollowing reasons.

One of the reasons is a device size. When the photodiode is to bedensely disposed in an array, the mesa size is limited. Namely, althoughthe line of electric force extending into an InGaAs side surface isreduced by the lower mesa near the side surface of the intermediate mesaof FIG. 8 (B-B′ cross section), a potential drop remains according tothe size of the lower mesa (FIG. 9(B)). When the photodiode is to bedensely disposed in an array, the size of the lower mesa is limited tomake the photodiode close, and it is difficult to completely suppressthe leakage current attributable to the InGaAs surface.

The other reason is a series resistance of the device. The seriesresistance is required to be reduced to realize the high-speed operationof the device. As shown in FIG. 8, in the structure in which the p-typeInP contact layer 605 is disposed in the upper portion, and the n-typeInP contact layer 602 is disposed in the lower portion, the size of theintermediate mesa less affects the series resistance. However, in theinverted photodiode disclosed in the Patent Document 1, the lowerportion is a p-type InP contact layer. Since the hall mobility of thep-type InP contact layer is small, if the size of the p-type InP contactlayer increases, the series resistance increases. When the seriesresistance is to be reduced to realize the high-speed operation, thesize of the p-type InP contact layer should be reduced, and thus it isdifficult to suppress the leakage current attributable to the InGaAssurface.

In the electroabsorption modulator aimed at high-speed operation, thereare the following reasons. In the electroabsorption modulator, a ridgetype optical waveguide is often configured, and a core layer regionincluding InGaAs is disposed in the portion 603 of FIG. 8 under a ridgewhich is an upper clad (equivalent to 605 of FIG. 8). In this structure,the potential drop remains on the mesa side surface of the pn junctionof a core, and the structure causes the occurrence of a leakage currentand deterioration of the device.

As described above, in the device configuration aimed at high-speedoperation, in terms of the size of the device and the series resistance,there is a problem that it is difficult to suppress the occurrence of aleakage current. Thus, in order to solve the problem, an object of thepresent invention is to provide a semiconductor device which can reducea device size, reduce a series resistance, and suppress a leakagecurrent.

Means for Solving the Problems

In order to achieve the above object, in the semiconductor deviceaccording to the present invention, an upper mesa smaller than anintermediate mesa is disposed on the intermediate mesa, and a non-dopedsemiconductor layer is disposed to cover an upper surface of theintermediate mesa. In this specification, a direction of stackingsemiconductor layers is sometimes described as a vertical direction, anda direction parallel to a substrate surface is sometimes described as ahorizontal direction. Layers close to a substrate are sometimesdescribed as lower layers, and layers far from the substrate aresometimes described as upper layers.

More specifically, the semiconductor device according to the presentinvention has a laminate structure including a first semiconductor layerprovided on one side of a substrate in parallel with the substratesurface, a p-type second semiconductor layer, an n-type thirdsemiconductor layer, and at least one of an n-type fourth semiconductorlayer and a p-type fifth semiconductor layer. In this semiconductordevice, the first semiconductor layer is disposed between the secondsemiconductor layer and the third semiconductor layer, and the impurityconcentration is lower than the impurity concentrations of the secondand third semiconductor layers. The fourth semiconductor layer isdisposed between the first semiconductor layer and the secondsemiconductor layer, and the bandgap is larger than that of the firstsemiconductor layer. The fifth semiconductor layer is disposed betweenthe first semiconductor layer and the third semiconductor layer, and thebandgap is larger than that of the first semiconductor layer. When thesecond semiconductor layer is far from the substrate relative to thethird semiconductor layer, the fourth semiconductor layer is essential,and the outer circumference of the second semiconductor layer is moreinward than the outer circumference of the fourth semiconductor layer.When the third semiconductor layer is far from the substrate relative tothe second semiconductor layer, the fifth semiconductor layer isessential, and the outer circumference of the third semiconductor layeris more inward than the outer circumference of the fifth semiconductorlayer.

The non-doped fourth or fifth semiconductor layer is inserted togenerate a potential difference in the fourth or fifth semiconductorlayer. Since the generation of the potential difference prevents avoltage from being generated on the side surface of the intermediatemesa, a leakage current can be suppressed. Thus, the horizontal size ofthe device can be reduced, and the series resistance can also be reducedby the size reduction.

Accordingly, this invention can provide a semiconductor device which canreduce a device size, reduce the series resistance, and suppress theleakage current.

In the semiconductor device according to this invention, in a case wherethe second semiconductor layer is far from the substrate relative to thethird semiconductor layer, when a reverse bias is applied to between thesecond semiconductor layer and the fourth semiconductor layer, apotential difference of not less than 0.2 V and not more than 1.0 V isgenerated in the fourth semiconductor layer. Further, in thesemiconductor device according to this invention, in a case where thethird semiconductor layer is far from the substrate relative to thesecond semiconductor layer, when a reverse bias is applied to betweenthe third semiconductor layer and the fifth semiconductor layer, apotential difference of not less than 0.2 V and not more than 1.0 V isgenerated in the fifth semiconductor layer. When the generated potentialdifference is larger than the above range, a minimum bias voltagecapable of maintaining the device operation increases. Meanwhile, whenthe generated potential difference is smaller than the above range,suppression of a leakage current becomes incomplete.

In the semiconductor device according to this invention, when the secondsemiconductor layer is far from the substrate relative to the thirdsemiconductor layer, an n-type sixth semiconductor layer adjacent to theopposite side of the fourth semiconductor layer of the secondsemiconductor layer is further provided. The semiconductor device havingthis structure can be used as an NPN transistor.

In the semiconductor device according to this invention, when the thirdsemiconductor layer is far from the substrate relative to the secondsemiconductor layer, a p-type seventh semiconductor layer adjacent tothe opposite side of the fifth semiconductor layer of the thirdsemiconductor layer is further provided. The semiconductor device havingthis structure can be used as a PNP transistor.

Advantageous Effects of the Invention

The present invention can provide a semiconductor device which canreduce a device size, reduce a series resistance, and suppress a leakagecurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining a semiconductor device according to thepresent invention. FIG. 1(A) is a cross-sectional view, and FIG. 1(B) isan upper view.

FIG. 2 is a view for explaining the semiconductor device according tothe present invention. FIG. 2(A) is a band diagram of an active portion,and FIG. 2(B) is a band diagram of a peripheral portion.

FIG. 3 is a view for explaining the semiconductor device according tothe present invention. FIG. 3(A) is a cross-sectional view, and FIG.3(B) is an upper view.

FIG. 4 is a view for explaining the semiconductor device according tothe present invention. FIG. 4(A) is a band diagram of an active portion,and FIG. 4(B) is a band diagram of a peripheral portion.

FIG. 5 is a cross-sectional view for explaining the semiconductor deviceaccording to the present invention.

FIG. 6 is a cross-sectional view for explaining a conventional HTB.

FIG. 7 is a band diagram in an A-A′ cross section of FIG. 6.

FIG. 8 is a cross-sectional view for explaining a conventionalphotodiode.

FIG. 9(A) is a band diagram in an A-A′ cross section of FIG. 8.

FIG. 9(B) is a band diagram in a B-B′ cross section of FIG. 8.

DESCRIPTION OF EMBODIMENTS

Hereinafter, although the present invention will be described in detailusing specific embodiments, the invention is not interpreted whilelimiting to the following description. Components denoted by the samereference numerals in the present specification and the drawingsmutually denote the same components.

Embodiment 1

FIGS. 1(A) to 2(B) are views for explaining a semiconductor device 10 ofthe embodiment 1. The semiconductor device 10 is an npn-typeheterostructure bipolar transistor (HBT). FIGS. 1(A) and 1(B) are viewsschematically showing a cross-sectional surface and an upper surface ofthe semiconductor device 10, respectively.

Reference numeral 101 is a semi-insulating InP substrate,

reference numeral 102 is an n-type InP subcollector layer (described asan n-InP subcollector layer),

reference numeral 103 is a low concentration of InGaAs collector layer,

reference numeral 104 is an n-type InGaAsP electric-field control layer(described as n-InGaAsP electric-field control layer),

reference numeral 105 is a low concentration of InGaAsP intermediatelayer,

reference numeral 106 is a high concentration of p-type InGaAs baselayer (described as p⁺-InGaAs base layer),

reference numeral 107 is an n-type InP emitter layer (described as n-InPemitter layer), and

reference numerals 108, 109, and 110 are an emitter electrode, a baseelectrode, and a collector electrode, respectively.

In the semiconductor device 10,

the InGaAs collector layer 103 is a first semiconductor layer,

the p⁺-InGaAs base layer 106 is a second semiconductor layer,

n-InP sub-collector layer 102 is a third semiconductor layer,

the InGaAsP intermediate layer 105 is a fourth semiconductor layer, and

the n-InP emitter layer 107 is a sixth semiconductor layer.

The low concentration in the InGaAs collector layer 103 and the InGaAsPintermediate layer 105 means “a state in which a donor or acceptor has aconcentration low enough to cause little or no generation of an electriccharge causing large electric field changes in the relevant layers whendepletion occurs”. Namely, the InGaAs collector layer 103 and theInGaAsP intermediate layer 105 each have a low donor or acceptorconcentration compared to any other doping layers, and even if thelayers are non-doped layers, the semiconductor device 10 can obtain theeffect of the present invention. The high concentration in the p⁺-InGaAsbase layer 106 means that “the acceptor concentration is high enough toallow ohmic contact with the base electrode 109”. For example, theimpurity concentration of the p⁺-InGaAs base layer 106 is preferably notless than 10¹⁹/cm³.

First, a method of manufacturing the semiconductor device 10 will bedescribed. In the manufacture of the semiconductor device 10, first,semiconductor layers 102 to 107 are epitaxially grown on asemi-insulating InP substrate 101 by an MO-VPE method. In themanufacture of the device, first, a mesa of the n-InP emitter layer 107,a mesa of the p⁺-InGaAs base layer 106, the low-concentration of InGaAsPintermediate layer 105, the n-InGaAsP electric-field control layer 104,a collector mesa including the InGaAs collector layer 103, and a mesa ofthe n-InP sub-collector layer 102 are sequentially formed as four stagesof mesas by a chemical etching method. When the p⁺-InGaAs base layer 106is processed, for the purpose of facilitating to stop etching at aninterface between the p⁺-InGaAs base layer 106 and the low concentrationof InGaAsP intermediate layer 105 under the p⁺-InGaAs base layer 106,the etching rate for the low concentration of InGaAsP intermediate layer105 is namely reduced relatively lower than that for the p⁺-InGaAs baselayer 106. More specifically, the bandgap energy of the lowconcentration of InGaAsP intermediate layer 105 is not less thanapproximately 1 eV.

Finally, the emitter electrode 108, the base electrode 109, and thecollector electrode 110 are formed. Although extraction electrodes,interlayer insulating layers, and pads thereof are not illustrated,these components are formed if necessary.

FIG. 1(B) is an upper view. A sub-collector mesa including the n-InPsub-collector layer 102, a collector mesa including the InGaAs collectorlayer 103 to the InGaAsP intermediate layer 105, a base mesa includingthe p⁺-InGaAs base layer 106, and an emitter mesa including the n-InPemitter layer 107 are formed from the InP substrate 101 side. The mesabecomes smaller as it is provided on the upper side, the outercircumference of the collector mesa is provided inside the outercircumference of the sub-collector mesa, the outer circumference of thebase mesa is provided inside the outer circumference of the collectormesa, and the outer circumference of the emitter mesa is provided insidethe base mesa. Although each mesa has a rectangular shape, the shape isnot limited to the rectangular shape.

When the emitter electrode 108, the base electrode 109, and thecollector electrode 110 of the semiconductor device 10 are subjected toapplication of a suitable bias voltage (typically, a base ofapproximately +0.6 to 0.8 V and a collector of approximately +0.3 to 3 Vfor the emitter) to be in an operating state, the band diagrams of anactive portion (A-A′ cross section) and a peripheral portion (B-B′ crosssection) of the base-collector junction are shown in FIGS. 2(A) and2(B), respectively.

In the manufacture of the semiconductor device, as shown in FIG. 2(A), adoping concentration of the n-InGaAsP electric field control layer 104is regulated so that a portion ranging from the p⁺-InGaAs base layer 106to a portion of the n-InP sub-collector layer 102 are depleted. Asdescribed later, the thickness of the n-InGaAsP electric field controllayer 104 is 20 to 40 nm, for example such that a potential leveldifference of an n₂-i-p portion is in a suitable range (0.2 V to 1.0 V),and the doping concentration of the n-InGaAsP electric field controllayer 104 is regulated to 2 to 5×10¹⁷/cm³.

In the semiconductor device 10, a conductivity type is arranged in suchan order that n₁ (n-type)-i(intrinsic)-n₂(n-type)-i(intrinsic)-p(p-type) are stacked from the substrate side. Itis important that the potential level difference at the n₂-i-p portionis in a suitable range (0.2 V to 1.0 V), and the potential leveldifference is usually preferably approximately 0.5 V to 0.8 V. If thepotential level difference is too large, a minimum bias voltage(base-collector voltage in the HBT) capable of maintaining the deviceoperation is increased. This voltage level difference is not necessaryfor the operation of HBT. The level difference does not significantlychange the response characteristics.

Meanwhile, in the B-B′ cross section of the peripheral portion, then-InP sub-collector layer 102 to the low concentration of the InGaAsPintermediate layer 105 are connected so that n₁-i-n₂-i, and there is nop-type layer (FIG. 2(B)). In such a state that a reverse bias voltage isapplied to the base-collector junction, even if the surface side of then-InGaAsP electric field control layer 104 is rather depleted, electronsstay in the lower portion, and a neutral layer remains. In order toperform screening the entrance of the electric field into the lowerportion, the potential drop of the low concentration of the InGaAscollector layer 103 does not occur. Namely, since a voltage is notapplied to the mesa side surface of the collector mesa (side surface ofthe InGaAs collector layer 103), the occurrence of a leakage current inthe forward direction and the opposite direction caused by the mesa sidesurface can be reduced.

Although the npn transistor structure has been described in theembodiment 1, the pnp transistor structure can be used by inverting theconductivity type.

Embodiment 2

FIGS. 3(A) to 4(B) are views for explaining a semiconductor device 20 ofthe embodiment 2. The semiconductor device 20 is a photodiode. FIGS.3(A) and 3(B) are views schematically showing a cross-sectional surfaceand an upper surface of the semiconductor device 20, respectively.

Reference numeral 201 is a semi-insulating InP substrate,

reference numeral 202 is a high concentration of p-type InGaAsPelectrode layer (described as a p⁺-InGaAsP electrode layer),

reference numeral 203 is a p-type InGaAs light-absorbing layer(described as a p-InGaAs light-absorbing layer),

reference numeral 204 is a low concentration of InGaAs light-absorbinglayer (described as a ud.-InGaAs light-absorbing layer),

reference numeral 205 is a p-type InGaAsP electric field control layer(described as an InGaAsP electric field control layer),

reference numeral 206 is a non-doped InGaAsP intermediate layer(described as a ud.-InGaAsP intermediate layer),

reference numeral 207 is an n-type InGaAsP electric field control layer(described as an n-InGaAsP electric field control layer),

reference numeral 208 is a low concentration of InGaAsP electron transitlayer (described as an n-InGaAsP electron transit layer),

reference numeral 209 is a high concentration of n-type InGaAsPelectrode layer (described as n⁺-InGaAsP electrode layer),

reference numeral 210 is a dielectric antireflection film,

reference numeral 211 is an n-electrode, and

reference numeral 212 is a p-electrode.

In the semiconductor device 20,

the ud.-InGaAs light-absorbing layer 204 is a first semiconductor layer,

the p-InGaAs light-absorbing layer 203 is a second semiconductor layer,

the n-InGaAsP electron transit layer 207 is a third semiconductor layer,and

the ud.-InGaAsP intermediate layer 206 is a fifth semiconductor layer.

The “low concentration” and “high concentration” have the same meaningsas those in the embodiment 1.

First, a method of manufacturing the semiconductor device 20 will bedescribed. In the manufacture of the semiconductor device 20, first,semiconductor layers 202 to 209 are epitaxially grown on asemi-insulating InP substrate 201 by the MO-VPE method. After that, anupper mesa constituted of the n⁺-InGaAsP electrode layer 209, then-InGaAsP electron transit layer 208, and the n-InGaAsP electric fieldcontrol layer 207 is formed by chemical etching method. Preferably, thecompositions of the n-InGaAsP electric field control layer 207 and theud.-InGaAsP intermediate layer 206 are changed, and the etching rate forthe upper layer is relatively increased, whereby etching is easilystopped near an interface between the both layers. After that, anintermediate mesa from the ud.-InGaAsP intermediate layer 206 to thep-InGaAs light-absorbing layer 203 is formed, and a lower mesa of thep⁺-InGaAsP electrode layer 202 is formed in a similar manner andelectrically separated. The upper mesa and the inside of theintermediate mesa under the upper mesa become an active portion (mainregion) of the device. After that, an n-electrode 211 and a p-electrode212 are formed. Although extraction electrodes, interlayer insulatinglayers, and pads thereof are not illustrated, these components areformed if necessary. Finally, a dielectric antireflection film 210 isformed.

FIG. 3(B) is an upper view. A lower mesa including the p⁺-InGaAsPelectrode layer 202, an intermediate mesa including the p-InGaAslight-absorbing layer 203 to the ud.-InGaAsP intermediate layer 206, andan upper mesa including the n-InGaAsP electric field control layer 207to the n⁺-InGaAsP electrode layer 209 are formed from the InP substrate201 side. The mesa becomes smaller as it is provided on the upper side,the outer circumference of the intermediate mesa is provided inside theouter circumference of the lower mesa, and the outer circumference ofthe upper mesa is provided on the outer circumference of theintermediate mesa. Although each mesa has a circular shape, the shape isnot limited to the circular shape.

The operating condition of the semiconductor device 20 is less differentfrom a usual pin-type photodiode. The band diagrams of an active portion(A-A′ cross section) and a peripheral portion (B-B′ cross section) atthe time when a bias voltage is applied to the n-electrode 211 and thep-electrode 212 are shown in FIGS. 4(A) and 4(B), respectively. Theactive portion of FIG. 4(A) is depleted. For example, the bias voltageis in a range of 1.5 to 4 V.

In the manufacture of the semiconductor device, as shown in FIG. 4(A), adoping concentration of the p-InGaAsP electric field control layer 205is regulated so that the layers ranging from the ud.-InGaAslight-absorbing layer 204 to the n-InGaAsP electron transit layer 208are depleted. As described later, the thickness of the n-InGaAsPelectric field control layer 205 is 20 to 40 nm such that a potentiallevel difference of a p-i-n portion is in a suitable range (0.2 V to 1.0V), and the doping concentration of the n-InGaAsP electric field controllayer 205 is regulated to 2 to 5×10¹⁷/cm³.

In the semiconductor device 20, the arrangement of a conductivity typeis p (p-type)-i (intrinsic)-n (n-type) including the p-InGaAsP electricfield control layer 205, the ud.-InGaAsP intermediate layer 206, and then-InGaAsP electric field control layer 207. It is important that thepotential level difference at the p-i-n portion is in a suitable range(0.2 V to 1.0 V), and the potential level difference is usuallypreferably approximately 0.5 V to 0.8 V. If the potential leveldifference is too large, a minimum bias voltage capable of maintainingthe device operation is increased. The potential level difference is notrequired for operation of a photodiode and does not exist in aconventional photodiode. In the semiconductor device 20, although thevoltage level difference is provided in order to obtain the followingeffects, the potential level difference does not significantly changethe response characteristics of the photodiode.

Meanwhile, the peripheral portion (B-B′ cross section of FIG. 3(A)) ofthe intermediate mesa has a p₁-i-p₂-i structure from the p-InGaAslight-absorbing layer 203 side, and there is no n-type layer (FIG.4(B)). In such a state that a reverse bias voltage is applied to ajunction, even if the surface side of the p-InGaAsP electric fieldcontrol layer 205 is rather depleted, as long as a hole remains in thelower portion and a neutral layer remains, in order to perform screeningthe entrance of the electric field into the lower portion, the potentialdrop of the ud.-InGaAs light-absorbing layer 204 does not occur. Namely,since a voltage is not applied to the mesa side surface of theintermediate mesa (side surface of the ud.-InGaAs light-absorbing layer204), the occurrence of a leakage current in an opposite directioncaused by the mesa side surface can be reduced.

The photodiode of the semiconductor device 20 is an inverted photodiodein which the p⁺-InGaAsP electrode layer 202 is disposed in a lower mesa.In the prior art, in this type of photodiode structure, a seriesresistance attributable to a resistance in a horizontal direction isrelatively high in the p⁺-InGaAsP electrode layer 202, and this tends toaffect high-speed operation. However, in the semiconductor device 20,since a voltage is not applied to the ud.-InGaAs light-absorbing layer204 of the intermediate mesa side surface, the horizontal size of theintermediate mesa can be reduced without causing the increase of abackward leakage current, so that the series resistance can be reduced.The size reduction of the intermediate mesa contributes to the sizereduction of the entire device, so that a distance between two or morephotodiodes can be reduced to bring the photodiodes close to each other,and the semiconductor device 20 can be densely disposed in an array.

Embodiment 3

FIG. 5 is a view for explaining a semiconductor device 30 of theembodiment 3. The semiconductor device 30 is a ridge optical waveguidetype of light modulator using an electroabsorption effect. FIG. 5 is aview schematically showing a cross section of the semiconductor device30.

Reference numeral 301 is an InP substrate as a semi-insulatingsubstrate,

reference numeral 302 is an optical clad (described as an n-InP opticalclad) serving as an n-electrode layer,

reference numeral 303 is an optical confinement layer (described as aud.-InGaAsP optical confinement layer),

reference numeral 304 is a core layer (constituted of an InGaAs/InGaAlAsmultiple quantum well) having the electroabsorption effect,

reference numeral 305 is an optical confinement layer (described as aud.-InGaAsP optical confinement layer),

reference numeral 306 is an n-type InGaAsP electric field control layer(described as an n-InGaAsP electric field control layer,

reference numeral 307 is a connection layer (described as a ud.-InGaAsPconnection layer),

reference numeral 308 is an optical clad (described as p-InP opticalclad),

reference numeral 309 is a p-type InGaAsP electrode layer (described asa p⁺-InGaAsP electrode layer),

reference numeral 310 is a p-electrode, and

reference numeral 311 is an n-electrode.

In the semiconductor device 30,

the core layer 304 and the ud.-InGaAsP optical confinement layer 305constitute a first semiconductor layer,

the p-InP optical clad 308 is a second semiconductor layer,

the ud.-InGaAsP optical confinement layer 303 is a third semiconductorlayer, and

the ud.-InGaAsP connection layer 307 is a fourth semiconductor layer.

In the semiconductor device 30, although the polarity of pn is differentfrom that of the semiconductor device 20 of the embodiment 2, thelaminate arrangement of the layers is similar to that of thesemiconductor device 20. In the semiconductor device 30, the ud.-InGaAslight-absorbing layer 204 and the p-InGaAsP electric field control layer207 of the semiconductor device 20 are replaced with the core layer 304and the p-InP optical clad 308 to function as an optical waveguide typeof light modulator, respectively.

The conductivity types of the n-InP optical clad 302 to the p-InPoptical clad 308 on the substrate side are n₁-i-n₂-i-p as in thesemiconductor device 10 of the embodiment 1. Namely, a structure similarto that of the semiconductor device 10 is incorporated as a core layerand an optical confinement layer of a ridge optical waveguide type oflight modulator.

An upper surface of the ud.-InGaAsP connection layer 307 has a portionwhere the p-InP optical clad 308 is disposed and an exposed portionwhere the p-InP optical clad 308 is not provided. A mesa of the exposedportion provided lower than the ud.-InGaAsP connection layer 307 has alaminate structure of n₁-i-n₂-i. Accordingly, in an operating state inwhich a reverse bias is applied to the device, a neutral region whereelectrons remain in the n-InGaAsP electric field control layer 306 isformed, and a voltage drop does not occur in the side surface of thecore layer 304 exposed on the mesa side surface, or the voltage drop isreduced. Consequently, the occurrence of a leakage current attributableto the mesa side surface can be suppressed.

(Effects of the Invention)

The pn junction including InGaAs having a small bandgap is employed invarious electronic devices and optical devices. However, when thosedevices are mesa type of devices for the purpose of high-speedoperation, there is such a restriction that the size of the device isrequired to be increased to solve a problem that a leakage current iseasily generated.

In this invention, the potential level difference (the fourth or fifthsemiconductor layer) normally unrequired for the device operation ispositively inserted in a device structure. The potential leveldifference has such a function that even if a semiconductor having asmall bandgap is exposed on a mesa side surface, a potential drop amountof the portion is suppressed, and a leakage current inconvenient fordevice operation can be reduced. This effect can be commonly obtainedfor a heterostructure bipolar transistor, a photodiode, anelectroabsorption modulator, and so on. In the photodiode, since theleakage current is alleviated, the device size can be reduced, so thatin addition to improvement of operating speed with a reduction in seriesresistance, it is advantageous that the device can be densely disposedin an array.

Other Embodiments

In the embodiments 1 to 3, although the semiconductor device structureusing InP, InGaAs, and InGaAsP as the semiconductor material of eachlayer has been described, the semiconductor material of each layer isnot limited to above, and the present invention can be similarly appliedto a device using other semiconductor materials.

REFERENCE SIGNS LIST

-   10, 20, 30 Semiconductor device-   50: HBT-   60: Photodiode-   101: InP substrate-   102: n-InP sub-collector layer-   103: InGaAs collector layer-   104: n-InGaAsP electric field control layer-   105: InGaAsP intermediate layer-   106: p⁺-InGaAs base layer-   107: n-InP emitter layer-   108: Emitter electrode-   109: Base electrode-   110: Collector electrode-   201: InP substrate-   202: p⁺-InGaAsP electrode layer-   203: p-InGaAs light-absorbing layer-   204: ud.-InGaAs light-absorbing layer-   205: p-InGaAsP electric field control layer-   206: ud.-InGaAsP intermediate layer-   207: n-InGaAsP electric field control layer-   208: n-InGaAsP electron transit layer-   209: n⁺-InGaAsP electrode layer-   210: Dielectric antireflection film-   211: n-electrode-   212: p-electrode-   301: InP substrate-   302: n-InP optical clad-   303: ud.-InGaAsP optical confinement layer-   304: Core layer-   305: ud.-InGaAsP optical confinement layer-   306: n-InGaAsP electric field control layer-   307: ud.-InGaAsP connection layer-   308: p-InP optical clad-   309: p⁺-InGaAsP electrode layer-   310: p-electrode-   311: n-electrode-   501: Semi-insulating InP substrate-   502: InP sub-collector layer-   503: n-type InGaAs collector layer-   506: p-type InGaAs base layer-   507: n-type InP emitter layer-   508: Emitter electrode-   509: Base electrode-   510: Collector electrode-   601: Semi-insulating InP-   602: n-type InP contact layer-   603: InGaAs light-absorbing layer-   604: InGaAsP surface cover layer-   605: p-type InP contact layer-   606: p-electrode-   607: n-electrode

What is claimed is:
 1. A semiconductor device comprising a laminatestructure comprising a first semiconductor layer provided on one side ofa substrate in parallel with a surface of the substrate surface, ap-type second semiconductor layer, an n-type third semiconductor layer,and at least one of an n-type fourth semiconductor layer and a p-typefifth semiconductor layer, wherein the first semiconductor layer isdisposed between the second semiconductor layer and the thirdsemiconductor layer and has an impurity concentration lower than theimpurity concentrations of the second and third semiconductor layers,the fourth semiconductor layer is disposed between the firstsemiconductor layer and the second semiconductor layer and has a bandgaplarger than that of the first semiconductor layer, the fifthsemiconductor layer is disposed between the first semiconductor layerand the third semiconductor layer and has a bandgap larger than that ofthe first semiconductor layer, when the second semiconductor layer isfar from the substrate relative to the third semiconductor layer, thefourth semiconductor layer is essential, the outer circumference of thesecond semiconductor layer is more inward than the outer circumferenceof the fourth semiconductor layer, when the third semiconductor layer isfar from the substrate relative to the second semiconductor layer, thefifth semiconductor layer is essential, and the outer circumference ofthe third semiconductor layer is more inward than the outercircumference of the fifth semiconductor layer.
 2. The semiconductordevice according to claim 1, wherein in a case where the secondsemiconductor layer is far from the substrate relative to the thirdsemiconductor layer, when a reverse bias is applied to between thesecond semiconductor layer and the fourth semiconductor layer, apotential difference of not less than 0.2 V and not more than 1.0 V isgenerated in the fourth semiconductor layer.
 3. The semiconductor deviceaccording to claim 1, wherein in a case where the third semiconductorlayer is far from the substrate relative to the second semiconductorlayer, when a reverse bias is applied to between the third semiconductorlayer and the fifth semiconductor layer, a potential difference of notless than 0.2 V and not more than 1.0 V is generated in the fifthsemiconductor layer.
 4. The semiconductor device according to claim 1,further comprising an n-type sixth semiconductor layer adjacent to theopposite side of the fourth semiconductor layer of the secondsemiconductor layer when the second semiconductor layer is far from thesubstrate relative to the third semiconductor layer.
 5. Thesemiconductor device according to claim 1 further comprising a p-typeseventh semiconductor layer adjacent to the opposite side of the fifthsemiconductor layer of the third semiconductor layer when the thirdsemiconductor layer is far from the substrate relative to the secondsemiconductor layer.
 6. The semiconductor device according to claim 2,further comprising an n-type sixth semiconductor layer adjacent to theopposite side of the fourth semiconductor layer of the secondsemiconductor layer when the second semiconductor layer is far from thesubstrate relative to the third semiconductor layer.
 7. Thesemiconductor device according to claim 3, further comprising a p-typeseventh semiconductor layer adjacent to the opposite side of the fifthsemiconductor layer of the third semiconductor layer when the thirdsemiconductor layer is far from the substrate relative to the secondsemiconductor layer.